Britain is to send extra troops to Afghanistan after efforts to persuade Nato allies to contribute more towards operations in the country failed, the defence secretary said tonight.

In a statement released by the Ministry of Defence, Des Browne said he would announce details of which forces were going, and how many, to parliament on Monday.

Nato has to act "or we will put at risk everything we have achieved across Afghanistan in the last five years", he said.

"We have been trying hard to get other nations to live up to the joint commitment Nato made to Afghanistan and provide more forces, forces which are authorised to fight. We will continue to press. But we must be realistic."

He added: "We are acutely aware that our armed forces, particularly the army, continue to operate at a high operational tempo, but we believe this additional commitment is manageable." The statement said a Nato meeting earlier this month in Seville, Spain, had tried to persuade member countries to send extra troops to the more dangerous areas in southern and eastern Afghanistan, but without success.

"The UK has always agreed that Nato needs more forces in these regions, where the Taliban challenge is most serious - in particular, for a greater manoeuvre capability," Mr Browne said.

The new forces will be sent to the south of the country, where 5,000 British troops, part of a Nato contingent of 35,000 are already based.

The existing UK forces are stationed in Helmand province, a former Taliban stronghold still responsible for 60% of Afghanistan's opium production.

Other Nato countries, including France and Germany, have faced increasing criticism for restricting their troops' operations to more peaceful areas of Afghanistan.

On Tuesday, the Foreign Office minister Kim Howells urged Britain's allies in Afghanistan to do more, telling parliament that some of their helicopters might as well be "parked up in leading European airports" for all the good they were doing.

Media reports today had said Mr Browne was expected to announce the dispatch of 1,000 extra troops on Monday. However, he said, such speculation was "likely to be causing concern among our forces and their families, and I feel obliged to clarify the situation".

The MoD was still working out the final details of the deployment, which was agreed by the cabinet yesterday, he said. Earlier today, the Taliban threatened the deadliest year yet for foreign troops in Afghanistan ahead of an expected spring offensive.

"This year will prove to be the bloodiest for the foreign troops. It is not just a threat: we will prove it," the senior Taliban commander Mullah Dadullah told the Reuters news agency via satellite phone.

He said militants would be armed with shipments of new weapons, including some capable of bringing down coalition helicopters. . "The Taliban's war preparations are going on in caves and in mountains. Our 6,000 fighters are ready for attacks on foreign troops after the change in weather and as it becomes warmer," he said.

The Taliban, which ruled Afghanistan until a US-led invasion of the country in late 2001, has said it will increase the use of suicide attacks after conventional battles brought heavy rebel losses last year.

The deepening of Britain's commitment comes a day after Tony Blair told MPs of plans to cut 1,600 British troops in southern Iraq by the summer, to roughly 7,100. Further reductions were planned later in the year.

The move also comes after the Italian coalition government led by Romano Prodi resigned on Tuesday after it lost a vote in parliament, largely over its plans to retain nearly 2,000 troops in Afghanistan.

The shadow defence secretary, Liam Fox, today accused the British government itself of failing to do enough to get help from other Nato members.

"Those troops should be coming from countries such as Germany, France, Italy and Spain, who have so far not shown the adequate resolve to be part of a full Nato complement in Afghanistan," he told BBC Radio 4's Today programme.

"It is clear that the government has failed to get our Nato allies to carry their share of the burden in Afghanistan."

Nato has been preparing for an increase in Taliban activity once the winter snows melt. It is also concerned that its plans to slash farmers' relatively remunerative poppy production could lead to a backlash.